1 / 39

CMPE 150 Fall 2005 Lecture 17

CMPE 150 Fall 2005 Lecture 17. Introduction to Computer Networks. Announcements. Midterm on 11.04. In class, closed books/notes. Homework 3 is up. Due on 11.07.05. Lab this week: discussion/review sessions for midterm. Today. MAC (cont’d). Types of MAC. Control: Distributed.

cecilia-silva
Télécharger la présentation

CMPE 150 Fall 2005 Lecture 17

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CMPE 150Fall 2005Lecture 17 Introduction to Computer Networks

  2. Announcements • Midterm on 11.04. • In class, closed books/notes. • Homework 3 is up. • Due on 11.07.05. • Lab this week: discussion/review sessions for midterm.

  3. Today • MAC (cont’d).

  4. Types of MAC • Control: • Distributed. • Centralized. • How they coordinate medium access: • Round-robin. • Scheduled-access. • Contention-based.

  5. Round-Robin MAC • Centralized: polling. • Distributed: token passing.

  6. Scheduled Access MAC • Time divided into slots. • Station reserves slots in the future. • Multiple slots for extended transmissions. • Suited to stream traffic.

  7. Scheduled-Access MAC: Example • The basic bit-map protocol.

  8. Contention-Based MAC • No control. • Stations try to acquire the medium. • Distributed in nature.

  9. MAC Protocols • Contention-based • ALOHA and Slotted ALOHA. • CSMA. • CSMA/CD. • Round-robin : token-based protocols. • Token bus. • Token ring.

  10. Contention-Based MACs • ALOHA family. • CSMA family.

  11. ALOHA Protocols: Performance • Throughput versus offered traffic for ALOHA systems.

  12. ALOHA Protocols: Summary • Simple. • But, poor utilization… • When?

  13. CSMA Protocols

  14. Carrier Sense Multiple Access • The capacity of ALOHA or slotted ALOHAis limited by the large vulnerability period of a packet. • By listening before transmitting, stations try to reduce the vulnerability period to one propagation delay. • This is the basis of CSMA (Kleinrock and Tobagi, UCLA, 1975).

  15. CSMA • Station that wants to transmit first listens to check if another transmission is in progress (carrier sense). • If medium is in use, station waits; else, it transmits. • Collisions can still occur. • Transmitter waits for ACK; if no ACKs, retransmits.

  16. Collisions • Can collisions still occur?

  17. CSMA Flavors • 1-persistent CSMA (IEEE 802.3) • If medium idle, transmit; if medium busy, wait until idle; then transmit with p=1. • If collision, waits random period and starts again. • Non-persistent CSMA: if medium idle, transmit; otherwise wait a random time before re-trying. • Thus, station does not continuously sense channel when it is in use. • P-persistent: when channel idle detected, transmits packet in the first slot with p. • Slotted channel, i.e., with probability q = p-1, defers to next slot.

  18. CSMA vesrsus Aloha • Comparison of the channel utilization versus load for various random access protocols.

  19. CSMA/CD • CSMA with collision detection. • Problem: when frames collide, medium is unusable for duration of both (damaged) frames. • For long frames (when compared to propagation time), considerable waste. • What if station listens while transmitting?

  20. CSMA/CD Protocol 1. If medium idle, transmit; otherwise 2. 2. If medium busy, wait until idle, then transmit with p=1. 3. If collision detected, transmit brief jamming signal and abort transmission. 4. After aborting, wait random time, try again.

  21. CSMA/CD Performance • Wasted capacity restricted to time to detect collision. • Time to detect collision < 2*maximum propagation delay. • Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission. • Thus frames need to be at least “2*RTT” long.

  22. CSMA with Collision Detection • CSMA/CD can be in one of three states: contention, transmission, or idle.

  23. Ethernet

  24. Ethernet • IEEE 802. family. • Standards for LANs and MANs. • Ethernet defined in the IEEE 802.3 standard. • PHY, MAC, and LLC.

  25. Where in the Stack? • (a) Position of LLC. (b) Protocol formats.

  26. Ethernet MAC • CSMA/CD. • Binary exponential back-off.

  27. Ethernet MAC (Cont’d)

  28. Ethernet Frame Length • At 10Mbps with 2,500 m maximum distance: • RTT ~ 50 microsec. • Thus, at least 500-bit frames. • It is actually 512 bits. • If fewer bits than that, add “padding”.

  29. Ethernet Frame • Frame formats. (a) DIX Ethernet, (b) IEEE 802.3. Destination address: “1” for “group” addresses. Type: mux/demux of network layer protocols. Data:max. of 1500 bytes.

  30. Binary Exponential Backoff • Randomization after collision.

  31. BEB (Cont’d) • After first collision, each station waits for 0 or 1 slot before trying again. • After second collision, they pick either 0, 1, 2, or 3 slots at random to wait. • After 3rd. collision, number of slots to wait is between 0 and 23 -1. • In general, after I collisions, wait is between 0 and 2i – 1. • After 10 collisions, randomization interval frozen at 1023 slots. • After 16 collisions, error!

  32. Ethernet Performance

  33. Ethernet Cabling • The most common kinds of Ethernet cabling.

  34. Ethernet Cabling (Cont’d) • Three kinds of Ethernet cabling. • (a) 10Base5, (b) 10Base2, (c) 10Base-T.

  35. Ethernet Topologies • Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

  36. Switched Ethernet • A simple example of switched Ethernet.

  37. Fast Ethernet • The original fast Ethernet cabling.

  38. Gigabit Ethernet • (a) A two-station Ethernet. (b) A multistation Ethernet.

  39. Gigabit Ethernet (Cont’d) • Gigabit Ethernet cabling.

More Related